Integrated quantum cold point coolers

ABSTRACT

A method for forming a thermoelement for a thermoelectric cooler is provided. In one embodiment a first substrate having a plurality of pointed tips separated by valleys wherein the substrate is covered by a metallic layer, portions of the metallic layer is covered by an insulator, and other portions of the metallic layer are exposed is formed. The other portions of the metallic layer that are exposed are covered with a thermoelectric material overcoat. A second substrate of thermoelectric material is then fused to the pointed tip side of the first substrate by, for example, heating the back of the first substrate to melt the thermoelectric material overcoat or by passing current through the pointed tips to induce Joule heating and thereby melt the thermoelectric material overcoat.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to devices for cooling substances such as,for example, integrated circuit chips, and more particularly, thepresent invention relates to thermoelectric coolers.

2. Description of Related Art

As the speed of computers continues to increase, the amount of heatgenerated by the circuits within the computers continues to increase.For many circuits and applications, increased heat degrades theperformance of the computer. These circuits need to be cooled in orderto perform most efficiently. In many low end computers, such as personalcomputers, the computer may be cooled merely by using a fan and fins forconvective cooling. However, for larger computers, such as main frames,that perform at faster speeds and generate much more heat, thesesolutions are not viable.

Currently, many main-frames utilize vapor compression coolers to coolthe computer. These vapor compression coolers perform essentially thesame as the central air conditioning units used in many homes. However,vapor compression coolers are quite mechanically complicated requiringinsulation and hoses that must run to various parts of the main frame inorder to cool the particular areas that are most susceptible todecreased performance due to overheating.

A much simpler and cheaper type of cooler are thermoelectric coolers.Thermoelectric coolers utilize a physical principle known as the PeltierEffect, by which DC current from a power source is applied across twodissimilar materials causing heat to be absorbed at the junction of thetwo dissimilar materials. Thus, the heat is removed from a hot substanceand may be transported to a heat sink to be dissipated, thereby coolingthe hot substance. Thermoelectric coolers may be fabricated within anintegrated circuit chip and may cool specific hot spots directly withoutthe need for complicated mechanical systems as is required by vaporcompression coolers.

However, current thermoelectric coolers are not as efficient as vaporcompression coolers requiring more power to be expended to achieve thesame amount of cooling. Furthermore, current thermoelectric coolers arenot capable of cooling substances as greatly as vapor compressioncoolers. Therefore, a thermoelectric cooler with improved efficiency andcooling capacity would be desirable so that complicated vaporcompression coolers could be eliminated from small refrigerationapplications, such as, for example, main frame computers, thermalmanagement of hot chips, RF communication circuits, magnetic read/writeheads, optical and laser devices, and automobile refrigeration systems.

SUMMARY OF THE INVENTION

The present invention provides a method for forming a thermoelement fora thermoelectric cooler. In one embodiment a first substrate having aplurality of pointed tips separated by valleys wherein the substrate iscovered by a metallic layer, portions of the metallic layer is coveredby an insulator, and other portions of the metallic layer are exposed isformed. The other portions of the metallic layer that are exposed arecovered with a thermoelectric material overcoat. A second substrate ofthermoelectric material is then fused to the pointed tip side of thefirst substrate by, for example, heating the back of the first substrateto melt the thermoelectric material overcoat or by passing currentthrough the pointed tips to induce Joule heating and thereby melt thethermoelectric material overcoat.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features believed characteristic of the invention are setforth in the appended claims. The invention itself, however, as well asa preferred mode of use, further objectives and advantages thereof, willbest be understood by reference to the following detailed description ofan illustrative embodiment when read in conjunction with theaccompanying drawings, wherein:

FIG. 1 depicts a high-level block diagram of a Thermoelectric Cooling(TEC) device in accordance with the prior art;

FIG. 2 depicts a cross sectional view of a thermoelectric cooler withenhanced structured interfaces in accordance with the present invention;

FIG. 3 depicts a planer view of thermoelectric cooler 200 in FIG. 2 inaccordance with the present invention;

FIGS. 4A and 4B depicts cross sectional views of tips that may beimplemented as one of tips 250 in FIG. 2 in accordance with the presentinvention;

FIG. 5 depicts a cross sectional view illustrating the temperature fieldof a tip near to a superlattice in accordance with the presentinvention;

FIG. 6 depicts a cross sectional view of a thermoelectric cooler withenhanced structured interfaces with all metal tips in accordance withthe present invention;

FIG. 7 depicts a cross-sectional view of a sacrificial silicon templatefor forming all metal tips in accordance with the present invention;

FIG. 8 depicts a flowchart illustrating an exemplary method of producingall metal cones using a silicon sacrificial template in accordance withthe present invention;

FIG. 9 depicts a cross sectional view of all metal cones formed usingpatterned photoresist in accordance with the present invention;

FIG. 10 depicts a flowchart illustrating an exemplary method of formingall metal cones using photoresist in accordance with the presentinvention;

FIG. 11 depicts a cross-sectional view of a thermoelectric cooler withenhanced structural interfaces in which the thermoelectric materialrather than the metal conducting layer is formed into tips at theinterface in accordance with the present invention;

FIG. 12 depicts a flowchart illustrating an exemplary method offabricating a thermoelectric cooler in accordance with the presentinvention;

FIG. 13 depicts a cross-sectional diagram illustrating the positioningof photoresist necessary to produce tips in a thermoelectric material;

FIG. 14 depicts a diagram showing a cold point tip above a surface foruse in a thermoelectric cooler illustrating the positioning of the tiprelative to the surface in accordance with the present invention;

FIG. 15 depicts a schematic diagram of a thermoelectric power generator;and

FIGS. 16A-16J depict cross sectional diagrams illustrating a process forfabricating thermoelements with pointed tip interfaces in accordancewith the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

With reference now to the figures and, in particular, with reference toFIG. 1, a high-level block diagram of a Thermoelectric Cooling (TEC)device is depicted in accordance with the prior art. Thermoelectriccooling, a well known principle, is based on the Peltier Effect, bywhich DC current from power source 102 is applied across two dissimilarmaterials causing heat to be absorbed at the junction of the twodissimilar materials. A typical thermoelectric cooling device utilizesp-type semiconductor 104 and n-type semiconductor 106 sandwiched betweenpoor electrical conductors 108 that have good heat conductingproperties. N-type semiconductor 106 has an excess of electrons, whilep-type semiconductor 104 has a deficit of electrons.

As electrons move from electrical conductor 110 to n-type semiconductor106, the energy state of the electrons is raised due to heat energyabsorbed from heat source 112. This process has the effect oftransferring heat energy from heat source 112 via electron flow throughn-type semiconductor 106 and electrical conductor 114 to heat sink 116.The electrons drop to a lower energy state and release the heat energyin electrical conductor 114.

The coefficient of performance, η, of a cooling refrigerator, such asthermoelectric cooler 100, is the ratio of the cooling capacity of therefrigerator divided by the total power consumption of the refrigerator.Thus the coefficient of performance is given by the equation:$\eta = \frac{{\alpha \quad {IT}_{c}} - {\begin{matrix}1 \\2\end{matrix}I^{2}R} - {K\quad \Delta \quad T}}{{I^{2}R} + {\alpha \quad I\quad \Delta \quad T}}$

where the term αIT_(c) is due to the thermoelectric cooling, the term½I²R is due to Joule heating backflow, the term KΔT is due to thermalconduction, the term I²R is due to Joule loss, the term αIΔT is due towork done against the Peltier voltage, α is the Seebeck coefficient forthe material, T_(c) is the temperature of the heat source, and ΔT is thedifference in the temperature of the heat source from the temperature ofthe heat sink.

The maximum coefficient of performance is derived by optimizing thecurrent, I, and is given by the following relation:$\eta_{\max} = {\left( \frac{T_{c}}{\Delta \quad T} \right)\left\lbrack \frac{\gamma  -^{T_{Is}}T_{c}}{\gamma + 1} \right\rbrack}$where$\gamma = \sqrt{1 + {\frac{\alpha^{2}\sigma}{\lambda}\left( \frac{T_{h} + T_{c}}{2} \right)}}$and $ɛ = \frac{\gamma  -^{T_{h}}T_{c}}{\gamma + 1}$

where ε is the efficiency factor of the refrigerator. The figure ofmerit, ZT, is given by the equation:${ZT} = \frac{\alpha^{2}\sigma \quad T}{\lambda}$

where λ is composed of two components: λ_(e), the component due toelectrons, and λ_(L), the component due to the lattice. Therefore, themaximum efficiency, ε, is achieved as the figure of merit, ZT,approaches infinity. The efficiency of vapor compressor refrigerators isapproximately 0.3. The efficiency of conventional thermoelectriccoolers, such as thermoelectric cooler 100 in FIG. 1, is typically lessthan 0.1. Therefore, to increase the efficiency of thermoelectriccoolers to such a range as to compete with vapor compressionrefrigerators, the figure of merit, ZT, must be increased to greaterthan 2. If a value for the figure of merit, ZT, of greater than 2 can beachieved, then the thermoelectric coolers may be staged to achieve thesame efficiency and cooling capacity as vapor compression refrigerators.

With reference to FIG. 2, a cross sectional view of a thermoelectriccooler with enhanced structured interfaces is depicted in accordancewith the present invention. Thermoelectric cooler 200 includes a heatsource 226 from which, with current I flowing as indicated, heat isextracted and delivered to heat sink 202. Heat source 226 may bethermally coupled to a substance that is desired to be cooled. Heat sink202 may be thermally coupled to devices such as, for example, a heatpipe, fins, and/or a condensation unit to dissipate the heat removedfrom heat source 226 and/or further cool heat source 226.

Heat source 226 is comprised of p− type doped silicon. Heat source 226is thermally coupled to n+ type doped silicon regions 224 and 222 oftips 250. N+ type regions 224 and 222 are electrical conducting as wellas being good thermal conductors. Each of N+ type regions 224 and 222forms a reverse diode with heat source 226 such that no current flowsbetween heat source 226 and n+ regions 224 and 222, thus providing theelectrical isolation of heat source 226 from electrical conductors 218and 220.

Heat sink 202 is comprised of p− type doped silicon. Heat sink 202 isthermally coupled to n+ type doped silicon region 204. N+ type region204 is electrically conducting and is a good thermal conductor. N+ typeregion 204 and heat sink 202 forms a reverse diode so that no currentflows between the N+ type region 204 and heat sink 202, thus providingthe electrical isolation of heat sink 202 from electrical conductor 208.More information about electrical isolation of thermoelectric coolersmay be found in commonly U.S. Pat. No. 6,222,113, the contents of whichare hereby incorporated herein for all purposes.

The need for forming reverse diodes with n+ and p− regions toelectrically isolate conductor 208 from heat sink 202 and conductors 218and 220 from heat source 226 is not needed if the heat sink 202 and heatsource 226 are constructed entirely from undoped non-electricallyconducting silicon. However, it is very difficult to ensure that thesilicon is entirely undoped. Therefore, the presence of the reversediodes provided by the n+ and p− regions ensures that heat sink 202 andheat source 226 are electrically isolated from conductors 208, 218, and220. Also, it should be noted that the same electrical isolation usingreverse diodes may be created other ways, for example, by using p+ typedoped silicon and n− type doped silicon rather than the p− and n+ typesdepicted. The terms n+ and p+, as used herein, refer to highly n dopedand highly p doped semiconducting material respectively. The terms n−and p−, as used herein, mean lightly n doped and lightly p dopedsemiconducting material respectively.

Thermoelectric cooler 200 is similar in construction to thermoelectriccooler 100 in FIG. 1. However, N-type 106 and P-type 104 semiconductorstructural interfaces have been replaced with superlattice thermoelementstructures 210 and 212 that are electrically coupled by doped region 204and electrical conductor 208. Electrical conductor 208 may be formedfrom platinum (Pt) or, alternatively, from other conducting materials,such as, for example, tungsten (W), nickel (Ni), or titanium coppernickel (Ti/Cu/Ni) metal films.

A superlattice is a structure consisting of alternating layers of twodifferent semiconductor materials, each several nanometers thick.Thermoelement 210 is constructed from alternating layers of N-typesemiconducting materials and the superlattice of thermoelement 212 isconstructed from alternating layers of P-type semiconducting materials.Each of the layers of alternating materials in each of thermoelements210 and 212 is approximately 10 nanometers (nm) thick. A superlattice oftwo semiconducting materials has lower thermal conductivity, λ, and thesame electrical conductivity, σ, as an alloy comprising the same twosemiconducting materials.

In one embodiment, superlattice thermoelement 212 comprises alternatinglayers of p-type bismuth chalcogenide materials such as, for example,alternating layers of Bi₂Te₃/Sb₂Te₃ with layers of Bi_(0.5)Sb_(1.5)Te₃,and the superlattice of thermoelement 210 comprises alternating layersof n-type bismuth chalcogenide materials, such as, for example,alternating layers of Bi₂Te₃ with layers of Bi₂Se₃. Other types ofsemiconducting materials may be used for superlattices forthermoelements 210 and 212 as well. For example, rather than bismuthchalcogenide materials, the superlattices of thermoelements 210 and 212may be constructed from cobalt antimony skutteridite materials.

Thermoelectric cooler 200 also includes tips 250 through whichelectrical current I passes into thermoelement 212 and then fromthermoelement 210 into conductor 218. Tips 250 includes n+ typesemiconductor 222 and 224 formed into pointed conical structures with athin overcoat layer 218 and 220 of conducting material, such as, forexample, platinum (Pt). Other conducting materials that may be used inplace of platinum include, for example, tungsten (W), nickel (Ni), andtitanium copper nickel (Ti/Cu/Ni) metal films. The areas between andaround the tips 250 and thermoelectric materials 210 and 212 should beevacuated or hermetically sealed with a gas such as, for example, drynitrogen.

On the ends of tips 250 covering the conducting layers 218 and 220 is athin layer of semiconducting material 214 and 216. Layer 214 is formedfrom a P-type material having the same Seebeck coefficient, α, as thenearest layer of the superlattice of thermoelement 212 to tips 250.Layer 216 is formed from an N-type material having the same Seebeckcoefficient, α, as the nearest layer of thermoelement 210 to tips 250.The P-type thermoelectric overcoat layer 214 is necessary forthermoelectric cooler 200 to function since cooling occurs in the regionnear the metal where the electrons and holes are generated. The n-typethermoelectric overcoat layer 216 is beneficial, because maximum coolingoccurs where the gradient (change) of the Seebeck coefficient ismaximum. The thermoelectric overcoats 214 and 216 are preferably in therange of 2-5 nanometers thick based upon present investigation.

By making the electrical conductors, such as, conductors 110 in FIG. 1,into pointed tips 250 rather than a planar interface, an increase incooling efficiency is achieved. Lattice thermal conductivity, λ, at thepoint of tips 250 is very small because of lattice mismatch. Forexample, the thermal conductivity, λ, of bismuth chalcogenides isnormally approximately 1 Watt/meter*Kelvin. However, in pointed tipstructures, such as tips 250, the thermal conductivity is reduced, dueto lattice mismatch at the point, to approximately 0.1Watts/meter*Kelvin. However, the electrical conductivity of thethermoelectric materials remains relatively unchanged. Therefore, thefigure of merit, ZT, may increased to greater than 2.5 for this kind ofmaterial. Another type of material that is possible for thesuperlattices of thermoelements 210 and 212 is cobalt antimonyskutteridites. These type of materials typically have a very highthermal conductivity, λ, making them normally undesirable. However, byusing the pointed tips 250, the thermal conductivity can be reduced to aminimum and produce a figure of merit, ZT, for these materials ofgreater than 4, thus making these materials very attractive for use inthermoelements 210 and 212.

Another advantage of the cold point structure is that the electrons areconfined to dimensions smaller than the wavelength (corresponding totheir kinetic energy). This type of confinement increases the localdensity of states available for transport and effectively increases theSeebeck coefficient. Thus, by increasing α and decreasing λ, the figureof merit ZT is increased.

Conventional thermoelectric coolers, such as illustrated in FIG. 1, arecapable of producing a cooling temperature differential, ΔT, between theheat source and the heat sink of around 60 Kelvin. However,thermoelectric cooler 200 is capable of producing a temperaturedifferential greater than 100 Kelvin. Thus, with two thermoelectriccoolers coupled to each other, cooling to temperatures in the range ofliquid Nitrogen (less than 100 Kelvin) is possible. However, differentmaterials may need to be used for thermoelements 210 and 212. Forexample, bismuth telluride has a very low α at low temperature (i.e.less than −100 degrees Celsius). However, bismuth antimony alloysperform well at low temperature.

Those of ordinary skill in the art will appreciate that the constructionof the thermoelectric cooler in FIG. 2 may vary depending on theimplementation taking into account the desired cooling, heat transfercapacity, current and voltage supplies. For example, more or fewer rowsof tips 250 may be included than depicted in FIG. 1. The depictedexample is not meant to imply architectural limitations with respect tothe present invention.

With reference now to FIG. 3, a planer view of thermoelectric cooler 200in FIG. 2 is depicted in accordance with the present invention.Thermoelectric cooler 300 includes an n-type thermoelectric materialsection 302 and a p-type thermoelectric material section 304. Bothn-type section 302 and p-type section 304 include a thin layer ofconductive material 306 that covers a silicon body.

Section 302 includes an array of conical tips 310 each covered with athin layer of n-type material 308 of the same type as the nearest layerof the superlattice for thermoelement 210. Section 304 includes an arrayof conical tips 312 each covered with a thin layer of p-type material314 of the same type as the nearest layer of the superlattice forthermoelement 212.

With reference now to FIGS. 4A and 4B, a cross sectional views of tipsthat may be implemented as one of tips 250 in FIG. 2 is depicted inaccordance with the present invention. Tip 400 includes a silicon cone402 that has been formed with a cone angle of approximately 35 degrees.A thin layer 404 of conducting material, such as platinum (Pt),overcoats the silicon 402. A thin layer of thermoelectric material 406covers the very end of the tip 400. The cone angle after all layers havebeen deposited is approximately 45 degrees. The effective point radiusof tip 400 is approximately 50 nanometers.

Tip 408 is an alternative embodiment of a tip, such as one of tips 250.Tip 408 includes a silicon cone 414 with a conductive layer 412 andthermoelectric material layer 410 over the point. However, tip 408 has amuch sharper cone angle than tip 400. The effective point radius of tip408 is approximately 10 nanometers. It is not known at this time whethera broader or narrower cone angle for the tip is preferable. In thepresent embodiment, conical angles of 45 degrees for the tip, asdepicted in FIG. 4A, have been chosen, since such angle is in the middleof possible ranges of cone angle and because such formation is easilyfabricated from silicon with a platinum overcoat. This is because a KOHetch along the <100> plane of silicon naturally forms a cone angle of 54degrees. Thus, after the conductive and thermoelectric overcoats havebeen added, the cone angle is approximately 45 degrees.

With reference now to FIG. 5, a cross sectional view illustrating thetemperature field of a tip near to a superlattice is depicted inaccordance with the present invention. Tip 504 may be implemented as oneof tips 250 in FIG. 2. Tip 504 has a effective tip radius at itssharpest point, a, of 10-50 nanometers. Thus, the temperature field islocalized to a very small distance, r, approximately equal to 2 a oraround 20-100 nanometers. Superlattice 502 need to be only a few layersthick to limit heat flow. Therefore, using pointed tips, athermoelectric cooler with only 5-10 layers for the superlattice issufficient.

Thus, fabricating a thermoelectric cooler, such as, for example,thermoelectric cooler 200, is simplified, since only a few layers of thesuperlattice must be formed. Also, thermoelectric cooler 200 can befabricated very thin (on the order of 100 nanometers thick) ascontrasted to conventional thermoelectric coolers which are on the orderof 3 millimeters or greater in thickness.

Other advantages of a thermoelectric cooler with pointed tip interfacesin accordance with the present invention include minimization of thethermal conductivity through the thermoelements, such as thermoelements210 and 212 in FIG. 2, because of the tip interfaces. Also, thetemperature/potential drops are localized to an area near the tips,effectively allowing scaling to sub-100-nanometer lengths. Furthermore,using pointed tips minimizes the number layers needed for superlatticethermoelements 210 and 212. The present invention also permitselectrodeposition of thin film structures. The smaller dimensions alsoallow for monolithic integration of n-type and p-type thermoelements.

The thermoelectric cooler of the present invention may be utilized tocool items, such as, for example, specific spots within a main framecomputer, lasers, optic electronics, photodetectors, and PCR ingenetics.

With reference now to FIG. 6, a cross sectional view of a thermoelectriccooler with enhanced structured interfaces with all metal tips isdepicted in accordance with the present invention. Although the presentinvention has been described above as having tips 250 constructed fromsilicon cones constructed from the n+ semiconducting regions 224 and222, tips 250 in FIG. 2 may be replaced by tips 650 as depicted in FIG.6. Tips 650 have all metal cones 618 and 620. In the depictedembodiment, cones 618 and 620 are constructed from copper and have anickel overcoat layer 660 and 662. Thermoelectric cooler 600 isidentical to thermoelectric cooler 200 in all other respects, includinghaving a thermoelectric overcoat 216 and 214 over the tips 650.Thermoelectric cooler 600 also provides the same benefits asthermoelectric cooler 200. However, by using all metal cones rather thansilicon cones covered with conducting material, the parasiticresistances within the cones become very low, thus further increasingthe efficiency of thermoelectric cooler 600 over the already increasedefficiency of thermoelectric cooler 200. The areas surrounding thecontact areas of tips 650 to thermoelectric materials 210 and 212 shouldbe vacuum or hermetically sealed with a low-thermal conductivity gas,such as, for example, argon.

Also, as in FIG. 2, heat source 226 is comprised of p− type dopedsilicon. In contrast to FIG. 2, however, silicon heat source 226 isthermally coupled to n+ type doped silicon regions 624 and 622 but doesnot form part of the tipped structure 650 as did silicon regions 224 and222 do in FIG. 2. N+ type doped silicon regions 624 and 622 do stillperform the electrical isolation function performed by regions 224 and222 in FIG. 2.

Several methods may be utilized to form the all metal cones as depictedin FIG. 6. For example, with reference now to FIG. 7, a cross-sectionalview of a sacrificial silicon template that may be used for forming allmetal tips is depicted in accordance with the present invention. Afterthe sacrificial silicon template 702 has been constructed having conicalpits, a layer of metal may be deposited over the template 702 to produceall metal cones 704. All metal cones 704 may then be used inthermoelectric cooler 600.

With reference now to FIG. 8, a flowchart illustrating an exemplarymethod of producing all metal cones using a silicon sacrificial templateis depicted in accordance with the present invention. To begin, conicalpits are fabricated by anisotropic etching of silicon to create a mold(step 802). This may be done by a combination of KOH etching, oxidation,and/or focused ion-beam etching. Such techniques of fabricating siliconpits are well known in the art.

The silicon sacrificial template is then coated with a thin sputteredlayer of seed metal, such as, for example, titanium (step 804). Next,copper is electrochemically deposited to fill the valleys (conical pits)in the sacrificial silicon template. (step 806). The top surface of thecopper is then planarized (step 808). Methods of planarizing a layer ofmetal are well known in the art. The silicon substrate is then removedby selective etching methods well known in the art (step 810). The allmetal cones produced in this manner may then be covered with a coat ofanother metal, such as, for example, nickel, and then with an ultra-thinlayer of thermoelectric material. The nickel overcoat aids inelectrodeposition of the thermoelectric material overcoat.

One advantage to this method of producing all metal cones is that thesilicon substrate mold is reusable if the copper is peeled from thesilicon substrate as the separation process. The silicon substrate moldmay be reused up to around 10 times before the mold degrades and becomesunusable.

Forming a template in this manner is very well controlled and producesvery uniform all metal conical tips since silicon etching is verypredictable and can calculate slopes of the pits and sharpness of thecones produced to a very few nanometers.

Other methods of forming all metal cones may be used as well. Forexample, with reference now to FIG. 9, a cross sectional view of allmetal cones 902 formed using patterned photoresist is depicted inaccordance with the present invention. In this method, a layer of metalis formed over the bottom portions of a partially fabricatedthermoelectric cooler. A patterned photoresist 904-908 is then used tofashion all metal cones 902 with a direct electrochemical etchingmethod. Often the tips are further sharpened by focused ion beammilling.

With reference now to FIG. 10, a flowchart illustrating an exemplarymethod of forming all metal cones using photoresist is depicted inaccordance with the present invention. To begin, small sections ofphotoresist are patterned over a metal layer, such as copper, of apartially fabricated thermoelectric cooler (step 1002). The photoresistmay be patterned in an array of sections having photoresist wherein eacharea of photoresist within the array corresponds to areas in which tipsto the all metal cones are desired to be formed. The metal is thendirectly etched electrochemically (step 1004) to produce cones 902 asdepicted in FIG. 9. The photoresist is then removed and the tips of theall metal cones may then be coated with another metal, such as, forexample, nickel (step 1006). The second metal coating over the all metalcones may then be coated with an ultra-thin layer of thermoelectricmaterial (step 1008). Thus, all metal cones with a thermoelectric layeron the tips may be formed which may used in a thermoelectric device,such as, for example, thermoelectric cooler 600. The all metal conicalpoints produced in this manner are not as uniform as those producedusing the method illustrated in FIG. 8. However, this method currentlyis cheaper and therefore, if cost is an important factor, may be a moredesirable method.

The depicted methods of fabricating all metal cones are merely examples.Other methods may be used as well to fabricate all metal cones for usewith thermoelectric coolers. Furthermore, other types of metals may beused for the all metal cone other than copper.

With reference now to FIG. 11, a cross-sectional view of athermoelectric cooler with enhanced structural interfaces in which thethermoelectric material rather than the metal conducting layer is formedinto tips at the interface is depicted in accordance with the presentinvention. Thermoelectric cooler 1100 includes a cold plate 1116 and ahot plate 1102, wherein the cold plate 1116 is in thermal contact withthe substance that is to be cooled. Thermal conductors 1114 and 1118provide a thermal couple between electrical conducting plates 1112 and1120 respectively. Thermal conductors 1114 and 1118 are constructed ofheavily n doped (n+) semiconducting material that provides electricalisolation between cold plate 1116 and conductors 1112 and 1120 byforming reverse biased diodes with the p− material of the cold plate1116. Thus, heat is transferred from the cold plate 1116 throughconductors 1112 and 1120 and eventually to hot plate 1102 from which itcan be dissipated without allowing an electrical coupling between thethermoelectric cooler 1100 and the substance that is to be cooled.Similarly, thermal conductor 1104 provides a thermal connection betweenelectrical conducting plate 1108 and hot plate 1102, while maintainingelectrical isolation between the hot plate and electrical conductingplate 1108 by forming a reverse biased diode with the hot plate 1102 p−doped semiconducting material as discussed above. Thermal conductor 1104is also an n+ type doped semiconducting material. Electrical conductingplates 1108, 1112, and 1120 are constructed from platinum (Pt) in thisembodiment. However, other materials that are both electricallyconducting and thermally conducting may be utilized as well. Also, itshould be mentioned that the areas surrounding tips 1130-1140 proximatethermoelectric materials 1122 and 1110 should be evacuated to produce avacuum or should be hermetically sealed with a low thermal conductivitygas, such as argon.

In this embodiment, rather than providing contact between thethermoelements and the heat source (cold end) metal electrode(conductor) through an array of points having metal in the pointelectrodes as in FIGS. 2 and 6, the array of points of contact betweenthe thermoelement and the metal electrode is provided by an array ofpoints 1130-1140 composed of thermoelements 1124 and 1126. The tips1130-1140 of the present embodiment may be formed from single crystal orpolycrystal thermoelectric materials by electrochemical etching.

In one embodiment, thermoelement 1124 is comprised of a super lattice ofsingle crystalline Bi₂Te₃/Sb₂Te₃ and Bi_(0.5)Sb_(1.5)Te₃ andthermoelement 1126 is formed of a super lattice of single crystallineBi₂Te₃/Bi₂Se₃ and Bi₂Te_(2.0)Se_(0.1). Electrically conducting plate1120 is coated with a thin layer 1122 of the same thermoelectricmaterial as is the material of the tips 1130-1134 that are nearest thinlayer 1120. Electrically conducting plate 1112 is coated with a thinlayer 1110 of the same thermoelectric material as is the material of thetips 1136-1140 that are nearest thin layer 1112.

With reference now to FIG. 12, a flowchart illustrating an exemplarymethod of fabricating a thermoelectric cooler, such as, for example,thermoelectric cooler 1100 in FIG. 11, is depicted in accordance withthe present invention. Optimized single crystal material is first bondedto a metal electrode 1301 by conventional means or the metal electrodeis deposited onto the single crystal material to form the electrodeconnection pattern (step 1202) as depicted in FIG. 13. The other side ofthe thermoelectric material 1314 is then patterned (step 1204) by usingphotoresist 1302-1306 as a mask and the metal electrode as an anode inan electrochemical bath to etch the surface (step 1206). The tips1308-1312 as depicted in FIG. 13 are formed by controlling and stoppingthe etching process at appropriate times.

A second single crystal substrate is thinned by chemical-mechanicalpolishing and then electrochemically etching the entire substrate tonanometer films (step 1210). The second ultra-thin substrate forms thecold end. The two substrates (the one with the ultra-thin thermoelectricmaterial and the other with the thermoelectric tips) are then clampedtogether with light pressure (step 1212). This structure retains highcrystallinity in all regions other than the interface at the tips. Also,similar methods can be used to fabricate polycrystalline structuresrather than single crystalline structures.

With reference now to FIG. 14, a diagram showing a cold point tip abovea surface for use in a thermoelectric cooler illustrating thepositioning of the tip relative to the surface is depicted in accordancewith the present invention. Although the tips, whether created in asall-metal or metal coated tips or as thermoelectric tips have beendescribed thus far as being in contact with the surface opposite thetips. However, although the tips may be in contact with the opposingsurface, it is preferable that the tips be very near the opposingsurface without fully touching the surface as depicted in FIG. 14. Thetip 1402 in FIG. 14 is situated near the opposing surface 1404 but isnot in physical contact with the opposing surface. Preferably, the tip1402 should be a distance d on the order of 1 nanometer or less from theopposing surface 1404. In practice, with a thermoelectric coolercontaining thousands of tips, some of the tips may be in contact withthe opposing surface while others are not in contact due to thedeviations from a perfect plane of the opposing surface.

By removing the tips from contact with the opposing surface, the thermalconductivity between the cold plate and the hot plate of athermoelectric cooler may be reduced. Electrical conductivity ismaintained, however, due to tunneling of electrons between the tips andthe opposing surface.

The tips of the present invention have also been described and depictedprimarily as perfectly pointed tips. However, as illustrated in FIG. 14,the tips in practice will typically have a slightly more rounded tip asis the case with tip 1402. However, the closer to perfectly pointed thetip is, the fewer number of superlattices needed to achieve thetemperature gradient between the cool temperature of the tip and the hottemperature of the hot plate.

Preferably, the radius of curvature r₀ of the curved end of the tip 1402is on the order of a few tens of nanometers. The temperature differencebetween successive layers of the thermoelectric material below surface1404 approaches zero after a distance of two (2) to three (3) times theradius of curvature r₀ of the end of tip 1402. Therefore, only a fewlayers of the super lattice 1406-1414 are necessary. Thus, asuperlattice material opposite the tips is feasible when the electricalcontact between the hot and cold plates is made using the tips of thepresent invention. This is in contrast to the prior art in which to usea superlattice structure without tips, a superlattice of 10000 or morelayers was needed to have a sufficient thickness in which to allow thetemperature gradient to approach zero. Such a number of layers wasimpractical, but using only 5 or 6 layers as in the present invention ismuch more practical.

Although the present invention has been described primarily withreference to a thermoelectric cooling device (or Peltier device) withtipped interfaces used for cooling, it will be recognized by thoseskilled in the art that the present invention may be utilized forgeneration of electricity as well. It is well recognized by thoseskilled in the art that thermoelectric devices can be used either in thePeltier mode (as described above) for refrigeration or in the Seebeckmode for electrical power generation. Referring now to FIG. 15, aschematic diagram of a thermoelectric power generator is depicted. Forease of understanding and explanation of thermoelectric powergeneration, a thermoelectric power generator according to the prior artis depicted rather than a thermoelectric power generator utilizing coolpoint tips of the present invention. However, it should be noted that inone embodiment of a thermoelectric power generator according to thepresent invention, the thermoelements 1506 and 1504 are replaced coolpoint tips, as for example, any of the cool point tip embodiments asdescribed in greater detail above.

In a thermoelectric power generator 1500, rather than running currentthrough the thermoelectric device from a power source 102 as indicatedin FIG. 1, a temperature differential, T_(H)−T_(L), is created acrossthe thermoelectric device 1500. Such temperature differential,T_(H)−T_(L), creates a current flow, I, as indicated in FIG. 15 througha resistive load element 1502. This is the opposite mode of operationfrom the mode of operation described in FIG. 1

Therefore, other than replacing a power source 102 with a load resistor1502 and maintaining heat elements 1512 and 1516 at differentialtemperatures T_(H) and T_(L) respectively with heat sources Q_(H) andQ_(L) respectively, thermoelectric device 1500 is identical incomponents to thermoelectric device 102 in FIG. 1. Thus, thermoelectriccooling device 1500 utilizes p-type semiconductor 1504 and n-typesemiconductor 1506 sandwiched between poor electrical conductors 1508that have good heat conducting properties. More information aboutthermoelectric electric power generation may be found in CRC Handbook ofThermoelectrics, edited by D. M. Rowe, Ph.D., D.Sc., CRC Press, N.Y.,(1995) pp. 479-488 and in Advanced Engineering Thermodynamics, 2ndEdition, by Adiran Bejan, John Wiley & Sons, Inc., N.Y. (1997), pp.675-682, both of which are hereby incorporated herein for all purposes.

With reference now to FIGS. 16A-16J, cross sectional diagramsillustrating a process for fabricating thermoelements with pointed tipinterfaces is depicted in accordance with the present invention. Thethermoelements fabricated with this method may be used as thermoelementsfor a thermoelectric cooler such as, for example, thermoelectric cooler200. To begin, a pointed tip substrate 1602 such as, for example, asilicon substrate or copper substrate peeled from silicon molds asdescribed above, is formed as depicted in FIG. 16A. Next, the pointedtip substrate 1602 is coated with a metal layer 1604, such as, forexample, titanium (Ti) or platinum (Pt), by, for example, a sputteringor an evaporation process, as depicted in FIG. 16B. A thin insulator1606, such as, for example, silicon dioxide, is deposited over the metallayer 1604 as depicted in FIG. 16C. The valleys between tips 1610-1612are filled with a sacrificial planarizing dielectric 1608 such that onlythe tips 1610-1612 of the metallic and insulator coated pointed tipsubstrate 1602 is exposed as depicted in FIG. 16D.

Next, the sacrificial dielectric 1608 and thin insulator 1606 are etchedtogether until the tips 1610-1612 are exposed as depicted in FIG. 16E. Athermoelectric material overcoat 1613-1615 is then selectively grown byelectrochemical methods or chemical vapor deposition (CVD) over the tips1610-1612 to a thickness of approximately five (5) nanometers asdepicted in FIG. 16F. The sacrificial dielectric 1608 is then removed asdepicted in FIG. 16G. The pointed tip substrate 1602 with pointed tips1610-1612 is mechanically aligned with a substantially flat surfacedthermoelectric substrate 1617 as depicted in FIG. 16H. The singlecrystal thermoelectric substrate 1617 is polished on one side 1619 andmetallized by sputter deposition of Ni 1618 on the opposite side. Theend of pointed tip substrate 1602 opposite pointed tips 1610-1612 isheated to approximately 550 degrees Celsius in order to melt the TEovercoats 1613-1615 and fuse the TE materials on the tips 1610-1612 tothermoelectric substrate 1617 as depicted in FIG. 16I. Alternatively, acurrent may be passed through the tips 1610-1612 to the point that Jouleheat melts the thermoelectric material 1613-1615 near the tips 1610-1612in order to fuse the tips 1610-1612 to thermoelectric substrate 1617.

The present invention has been described primarily with reference toconically shaped tips, however, other shapes of tips may be utilized aswell, such as, for example, pyramidically shaped tips. In fact, theshape of the tip does not need to be symmetric or uniform as long as itprovides a discrete set of substantially pointed tips through whichelectrical conduction between the two ends of a thermoelectric coolermay be provided. The present invention has applications to use in anysmall refrigeration application, such as, for example, cooling mainframe computers, thermal management of hot chips and RF communicationcircuits, cooling magnetic heads for disk drives, automobilerefrigeration, and cooling optical and laser devices.

The description of the present invention has been presented for purposesof illustration and description, but is not intended to be exhaustive orlimited to the invention in the form disclosed. Many modifications andvariations will be apparent to those of ordinary skill in the art. Theembodiment was chosen and described in order to best explain theprinciples of the invention, the practical application, and to enableothers of ordinary skill in the art to understand the invention forvarious embodiments with various modifications as are suited to theparticular use contemplated.

What is claimed is:
 1. A method for forming a thermoelement for athermoelectric cooler, the method comprising: forming a first substratehaving a plurality of pointed tips covered by a metallic layer, portionsof the metallic layer being covered by an insulating material, and otherportions of the metallic layer being exposed; covering portions of themetallic layer that are exposed with a thermoelectric material overcoat;and fusing a second substrate of thermoelectric material to thethermoelectric material overcoat.
 2. The method as recited in claim 1,wherein forming a substrate having a plurality of pointed tipscomprises: forming a substrate having a plurality of pointed tipsseparated by valleys; coating the substrate with a layer of metal;coating the layer of metal with a layer of insulating material; fillingthe valleys with a sacrificial material; and removing sacrificial andinsulating material to expose the plurality of tips.
 3. The method asrecited in claim 1, wherein fusing the second substrate ofthermoelectric material to the pointed tips comprises melting thethermoelectric material overcoat.
 4. The method as recited in claim 3,wherein the thermoelectric material overcoat is melted by heating thefirst substrate.
 5. The method as recited in claim 4, wherein the firstsubstrate is heated to approximately 550 degrees Celsius.
 6. The methodas recited in claim 3, wherein the thermoelectric material overcoat ismelted by passing a current through the tips in order to induce Jouleheating of the thermoelectric material overcoat.
 7. A system for forminga thermoelement for a thermoelectric cooler, the system comprising:means for forming a first substrate having a plurality of pointed tipscovered by a metallic layer, portions of the metallic layer beingcovered by an insulating material, and other portions of the metalliclayer being exposed; means for covering portions of the metallic layerthat are exposed with a thermoelectric material overcoat; and means forfusing a second substrate of thermoelectric material to thethermoelectric material overcoat.
 8. The system as recited in claim 7,wherein forming a substrate having a plurality of pointed tips separatedby valleys comprises: means for forming a substrate having a pluralityof pointed tips separated by valleys; means for coating the substratewith a layer of metal; means for coating the layer of metal with a layerof insulating material; means for filling the valleys with a sacrificialmaterial; and means for removing sacrificial and insulating material toexpose the plurality of tips.
 9. The system as recited in claim 7,wherein fusing the second substrate of thermoelectric material to thepointed tips comprises melting the thermoelectric material overcoat. 10.The system as recited in claim 9, wherein the thermoelectric materialovercoat is melted by heating the first substrate.
 11. The system asrecited in claim 10, wherein the first substrate is heated toapproximately 550 degrees Celsius.
 12. The system as recited in claim 9,wherein the thermoelectric material overcoat is melted by passing acurrent through the tips in order to induce Joule heating of thethermoelectric material overcoat.
 13. A method for forming athermoelement for use in a thermoelectric cooler, the method comprising:forming a first substrate having a plurality of pointed tips coveredwith a thermoelectric overcoat; fusing a second substrate ofthermoelectric material to the pointed tips of the first substrate. 14.The method as recited in claim 13, wherein fusing the second substratecomprises: mechanically aligning the second substrate to the pointedtips; and melting the thermoelectric overcoat.
 15. The method as recitedin claim 14, wherein the thermoelectric overcoat is melted by heatingthe first substrate.
 16. The method as recited in claim 15, wherein thefirst substrate is heated to approximately 550 degrees Celsius.
 17. Themethod as recited in claim 14, wherein the thermoelectric overcoat ismelted by passing a electric current through the pointed tips to induceJoule heating of the thermoelectric overcoat.
 18. A thermoelectriccooler, comprising: a first substrate having a plurality of pointedtips, the apexes of the tips having a selectively deposited overcoatthermoelectric material; a second substrate of planar thermoelectricmaterial; and fused connections between the overcoat thermoelectricmaterial and the planar thermoelectric material.
 19. The thermoelectriccooler as recited in claim 18, wherein the planar thermoelectricmaterial comprises a super lattice material.